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Mortgage Calculator with Extra Payments

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Estimate how regular or one-off overpayments could change the time it takes to repay a mortgage and the interest paid, using the interest rate and term entered.

Mortgage details

20% of property price
%
Enter the annual interest rate shown in your mortgage illustration or example scenario.
years
Enter the number of whole years for the mortgage term.
Payment frequency

This calculator models a repayment mortgage. It does not model interest-only repayment strategies.

Overpayments (optional)
per month

Regular and one-off overpayments are applied to the loan balance after the scheduled payment for that period.

Check your mortgage terms before overpaying. Some mortgages limit overpayments or charge an early-repayment charge if you repay more than your allowance or repay early.

Loan amount
£600,000
Scheduled payment
£3,595.49per month
No overpayment entered

Add a regular or one-off overpayment to compare this illustration with the standard repayment schedule.

Estimated annual repayment breakdown (standard schedule)
YearScheduled paymentExtra paymentsPrincipal repaidInterest paidBalance
1£43,146—£11,930£31,216£588,070
2£43,146—£12,572£30,574£575,498
3£43,146—£13,248£29,898£562,250
4£43,146—£13,960£29,185£548,290
5£43,146—£14,711£28,435£533,578
6£43,146—£15,502£27,643£518,076
7£43,146—£16,336£26,810£501,740
8£43,146—£17,215£25,931£484,525
9£43,146—£18,141£25,005£466,384
10£43,146—£19,116£24,030£447,268
11£43,146—£20,144£23,001£427,123
12£43,146—£21,228£21,918£405,896
13£43,146—£22,369£20,776£383,526
14£43,146—£23,573£19,573£359,954
15£43,146—£24,840£18,305£335,113
16£43,146—£26,176£16,970£308,937
17£43,146—£27,584£15,562£281,353
18£43,146—£29,068£14,078£252,285
19£43,146—£30,631£12,515£221,654
20£43,146—£32,278£10,867£189,376
21£43,146—£34,014£9,132£155,361
22£43,146—£35,844£7,302£119,518
23£43,146—£37,771£5,374£81,746
24£43,146—£39,803£3,343£41,944
25£43,146—£41,944£1,202£0

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How to use this calculator

Pick your currency, then enter the property price, your deposit, the annual interest rate and the mortgage term. Choose whether payments are made monthly or yearly. The calculator first builds a standard repayment schedule from those figures.

In the Overpayments (optional) section, add a regular extra amount for each period, a one-off lump sum, or both. When you enter a one-off amount, a field appears for the period it is made in — a number of months when payments are monthly, or years when they are yearly. Leave both overpayment fields blank to see the standard schedule on its own.

The result shows the estimated earlier repayment, the modeled payoff time and a comparison of interest paid with and without the overpayments. The Breakdown tab lists the annual schedule, the Chart tab plots both balance paths, and the Summary tab lists the assumptions used. This is a what-if illustration, not mortgage advice: it does not assess affordability, calculate early-repayment charges or tell you whether your lender allows an overpayment.

What do I need to enter?

  • Property price and Deposit — the loan is the price minus the deposit, so the deposit must be below the price.
  • Annual interest rate — 0% to 100%, held constant for the whole term.
  • Mortgage term — whole years from 1 to 50.
  • Payment frequency — Monthly or Yearly. It sets the period that the overpayment amounts and the one-off timing refer to.
  • Regular overpayment — an extra amount paid every period on top of the scheduled payment. Blank means none.
  • One-off overpayment — a single lump sum, blank for none. Once it is entered, "Make one-off overpayment after" asks for the whole number of months (or years, on yearly payments) within the term.

What do the results show?

  • Estimated earlier repayment — how much sooner the loan clears with your overpayments, shown once at least one is entered.
  • Comparison — the scheduled payment, your overpayments, the estimated payoff time, interest paid and the remaining balance after up to five years, with and without overpayments.
  • Interest difference — how much less interest you pay with the overpayments, or no difference at a 0% rate.
  • Breakdown, Chart and Summary tabs — the annual schedule, both balance paths over time, and the assumptions used.

How mortgage overpayments are calculated

The calculator first creates a standard repayment schedule from the property price, deposit, interest rate, term and payment frequency. It then creates a second schedule using the same scheduled repayment and applies any extra payments to the balance after the scheduled payment for each period.

  • P — loan amount, the property price minus the deposit
  • r — the annual interest rate you enter, as a decimal (5.25% is 0.0525)
  • m — payments per year: 12 for monthly or 1 for yearly
  • i = r / m — interest rate per payment period
  • n = T × m — original number of scheduled payments, for a term of T years
  • M — the standard scheduled repayment, which does not change
  • E — the regular overpayment each period (zero if none)
  • L — the one-off overpayment (zero if none)
  • k — the period the one-off overpayment is made in
  • Bt — the balance at the end of period t, starting from B₀ = P
M = P × i(1 + i)ⁿ ÷ ((1 + i)ⁿ − 1)
  • If i = 0, M = P ÷ n
  • interestₜ = Bₜ₋₁ × i
  • actual overpaymentₜ = min(scheduled extraₜ, balance after the scheduled payment)
  • Bₜ = balance after the scheduled payment − actual overpaymentₜ

For each period, interest is charged on the outstanding balance, the scheduled payment M is taken (reduced on the final period so the balance cannot go below zero), and then the extra payment — E, plus L in period k — is applied to the remaining balance. The extra payment is capped so the balance never becomes negative, and no further payments are charged once the balance reaches zero. The scheduled payment M is never recalculated in response to overpayments.

With a £600,000 loan at 5.25% over 25 years paid monthly, M is about £3,595.49. Adding £250 a month and a £10,000 lump sum after 12 months clears the modeled balance several years early and lowers the total interest charged in this constant-rate illustration. At a 0% rate, M is simply P ÷ n, every period’s interest is zero, and extra payments still shorten the modeled payoff time — but there is no interest difference to show.

Selecting a currency changes the labels and number formatting only. It does not convert amounts using exchange rates.

What this calculator does not do

It is an illustrative what-if calculation. It does not recommend overpaying, assess affordability, predict whether a lender will accept an overpayment, calculate early-repayment charges, apply an assumed overpayment allowance, recommend a mortgage term or product, or forecast interest rates. To compare overpaying with investing the same money instead, see the Compound Interest Calculator. Your mortgage offer and your lender’s rules determine whether overpayments are allowed, how they are applied and whether any charge applies.

Overpaying is not the only way to clear a mortgage faster. The Biweekly Mortgage Calculator models a schedule of half-payments every two weeks, and the 15 vs 30 Year Mortgage Calculator compares choosing a shorter term outright.

Frequently asked questions

What is a mortgage overpayment?

A mortgage overpayment is any payment you make on top of the scheduled monthly or yearly repayment your lender collects. It can be a regular extra amount each period or a single one-off lump sum. In this illustration an overpayment is applied to the loan balance after the scheduled payment for that period, so it reduces the capital you owe. Whether your lender allows overpayments, and how much, is set by your mortgage offer and terms.

Does this calculator reduce my payment or shorten the term?

It models shortening the term. The scheduled repayment stays exactly as calculated from the price, deposit, rate and term you entered, and the extra payments reduce the balance faster, so the modeled mortgage is repaid sooner. Some lenders instead let you keep the term and lower the scheduled payment after an overpayment — this calculator does not model that option.

Why does the calculator keep the scheduled repayment unchanged?

Keeping the scheduled repayment fixed is what turns an overpayment into a shorter term rather than a smaller bill. The calculator charges interest on the outstanding balance each period, takes the same scheduled payment, then applies your extra payment to whatever balance is left. Because the balance falls faster, less interest is charged over the life of the loan and the balance reaches zero earlier.

Can an overpayment trigger an early-repayment charge?

It can. Many fixed and discounted mortgages allow overpayments only up to a yearly limit and charge an early-repayment charge (ERC) on anything above it, or on repaying the mortgage in full during the deal period. This calculator does not calculate ERCs, does not assume any overpayment allowance and cannot tell you whether a planned overpayment is permitted. Check your mortgage offer and terms, or ask your lender, before overpaying.

Are overpayment allowances and ERCs included?

No. The illustration excludes early-repayment charges, overpayment allowances, product and arrangement fees, insurance, taxes, payment holidays, offset features and any lender-specific rules. It also assumes the interest rate you enter stays the same for the whole term. Your lender’s figures are the ones that apply.

What happens if a one-off payment is larger than the remaining balance?

The calculator caps the one-off payment at the balance left after that period’s scheduled payment, so the mortgage is simply repaid in that period and the balance never goes negative. The results then show the actual amount applied, not the larger amount you entered. If regular overpayments repay the mortgage before the one-off period, the one-off payment is not applied at all and a note says so.

Why might my lender’s figures differ?

Lenders use their own day-count and rounding conventions, recalculate interest daily or monthly, and may apply overpayments on a specific date rather than immediately. Your actual rate is also likely to change when a fixed, tracker or discounted period ends. This tool uses one constant rate and applies each extra payment straight after the scheduled payment, so treat its output as a rough comparison, not a statement of what your lender will do.

Does choosing another currency convert mortgage amounts?

No. Selecting a currency changes the currency symbol and number formatting on every field and result and nothing else. The price, deposit, loan amount, payments and schedule keep the same nominal numbers — 600,000 stays 600,000 whether it is shown as pounds, dollars or euros. There is no exchange-rate conversion.

Last updated 25 October 2026.